1. Field of the Invention
The present invention relates to an abnormality diagnosis apparatus of an evaporated gas purge system for diagnosing presence or absence of abnormality of an evaporated gas purge system for purging (discharging) evaporated gas (evaporated fuel) produced by evaporating fuel in a fuel tank to an intake pipe of an internal combustion engine.
2. Description of Related Art
Conventionally, in an evaporated gas purge system, in order to prevent evaporated gas generated from inside of a fuel tank from leaking out into the atmosphere, evaporated gas in the fuel tank is adsorbed in a canister via an evaporated gas passage, a purge control valve is installed at a midway of a purge passage for purging evaporated gas adsorbed in the canister to an intake pipe of an internal combustion engine and the purge control valve is controlled to open or close in accordance with an operating state of the internal combustion engine by which a purge flow rate of evaporated gas for purging from the canister to the intake pipe is controlled. In order to prevent an abnormality where evaporated gas leaks from the evaporated gas purge system to the atmosphere from being left for a long period of time, leakage of evaporated gas needs to detect at an early stage.
According to a conventional general method of abnormality diagnosis, a pressure sensor for detecting pressure in an evaporated gas system from a fuel tank to a purge control valve is installed, negative pressure (pressure at an intake pipe) is introduced into the evaporated gas system by opening the purge control valve and thereafter, the purge control valve is closed and a change in the pressure at inside of the evaporated gas system is detected by a pressure sensor in a state where the evaporated gas system is hermetically sealed and leakage of the evaporated gas system is detected by a degree of the change in the pressure.
However, the change in the pressure at inside of the evaporated gas system under the abnormality diagnosis, is significantly influenced by an amount of generating evaporated gas evaporated from fuel in the fuel tank and the amount of generating evaporated gas is changed by temperature of fuel, properties of fuel and so on and therefore, according to the conventional general method of abnormality diagnosis, it is impossible to determine whether the change in the pressure in the evaporated gas system is caused by leakage of the evaporated gas system or generation of evaporated gas and leakage of the evaporated gas system cannot be detected accurately.
In order to resolve the problem, as disclosed in Japanese Patent Application Laid-Open No. 5-125997, there has been a technology in which the atmosphere is introduced into an evaporated gas system, a pressure change amount .DELTA.P1 in the evaporated gas system in respect of the atmospheric pressure is detected in a state where the evaporated gas system is hermetically sealed, thereafter, a purge control valve is temporarily opened, negative pressure is introduced into the evaporated gas system, a pressure change amount .DELTA.P2 in the evaporated gas system in respect of the negative pressure is detected and leakage of the evaporated gas system is diagnosed by comparing the pressure change amounts .DELTA.P1 and .DELTA.P2 in the two detecting operation.
According to the abnormality diagnosing method of the publication, after detecting the pressure change amount .DELTA.P1 at a first time and before detecting the pressure change amount .DELTA.P2 at a second time, the negative pressure is introduced into the evaporated gas system by temporarily opening a purge control valve and accordingly, strong air flow is caused in the evaporated gas system by introducing the negative pressure. By the strong air flow, evaporated gas adsorbed in the canister flows out or a change in temperature at inside of a fuel tank or a change in concentration of evaporated gas is caused and accordingly, the evaporated gas condition in detecting the pressure change amount .DELTA.P2 at the second time differs from the evaporated gas condition in detecting the pressure change amount .DELTA.P1 at the first time. Even when the pressure change amounts .DELTA.P1 and .DELTA.P2 which are detected under different evaporated gas conditions are compared with each other, the influence of the change in the pressure caused by generation of evaporated gas cannot be canceled out and leakage of the evaporated gas system cannot be diagnosed accurately.
Further, according to the conventional apparatus mentioned above, when the inside of a hermetically-sealed section is adjusted to a predetermined negative pressure level, fuel gas is abruptly generated immediately after adjustment of pressure. Accordingly, even when rubber hose or the like forming a purge passage is not distracted and no leakage of fuel is caused, an amount of change in the pressure state is comparatively enlarged. Meanwhile, when a hole having a very small diameter (for example, hole of about diameter .phi.0.5 mm) is perforated in the rubber hose, the amount of change in the pressure caused by the leakage is comparatively reduced since the amount of leakage of fuel gas is comparatively small.
According to the conventional device in which presence or absence of abnormality is detected by the amount of change in the pressure from a predetermined negative pressure level to a side of positive pressure in the hermetically-sealed section, there poses a problem from reason mentioned above where even when a very small hole is perforated, it is difficult to discriminate whether the change in the pressure state in this occasion is caused by generation of a large amount of fuel gas immediately after the pressure adjustment or by the very small hole.
Further, when, for example, an automobile is running on an unpaved deteriorated road, curving or abruptly stopped, fuel in a fuel tank is rocked and the pressure change state is unpreparedly varied due to the rocking. Therefore, occurrence of abnormality in the fuel gas emission preventing apparatus may erroneously be detected.